Diffusion Driven by Partial Pressure Gradient

Steps:

• Note partial pressure of O2 in alveoli (14 kPa) exceeds that in capillaries (6 kPa).
• Recall gases diffuse from higher to lower partial pressure regions.
• Conclude net O2 diffusion occurs from alveoli to capillaries.

Why B is correct:

• Fick's law of diffusion states gases move down partial pressure gradients, so O2 flows from 14 kPa (alveoli) to 6 kPa (capillaries).

Why the others are wrong:

• A: Irrelevant to O2; no CO2 data provided.
• C: Reverses gradient; diffusion favors alveoli to capillaries.
• D: Unequal pressures create directional net flow, not equilibrium.

Final answer: B